1. Field of the Invention
The present invention relates to an alignment apparatus for correctly aligning a mask or reticle with a photosensitive substrate (wafer) in a projection exposure apparatus for optically transferring a pattern of an integrated circuit (IC) or the like.
2. Description of the Prior Art
Large Scale Integrated (LSI) circuit patterns are currently being further micronized. In order to meet demand for further micronization, a reducing projection exposure apparatus is used as a circuit pattern printing apparatus having high productivity. In a conventional apparatus of this type, a reticle pattern having a size several times (e.g., 5 times) that of a pattern to be printed on a silicon wafer is projected in a reduced size through a projection lens. An area of a wafer which can be printed from a single exposure operation is smaller than a square having a diagonal of 21 mm. For this reason, in order to print a pattern on the entire surface of a wafer having a diameter of about 125 mm, the so-called step-and-repeat method is adopted wherein a wafer is placed on s stage and is exposed upon each unit displacement.
In the manufacture of LSIs, more than one pattern layers are sequentially formed on a wafer. However, unless pattern misalignment (positional error) between different pattern layers is kept below a predetermined value, desired conductivity or insulation properties between the respective layers cannot be obtained, and the obtained LSI will not function normally. Misalignment of at most about 0.2 .mu.m is allowed for a minimum line width of 1 .mu.m.
In a reducing projection exposure apparatus, the off-axis system and the through-the-lens (TTL) system are known for aligning a pattern already formed on a wafer and a projection image of a reticle. In the off-axis system, the position of a wafer alignment mark is detected by an alignment microscope arranged outwardly of a projection lens, and the wafer is aligned with a reticle in one step. Therefore, the off-axis system allows fast alignment and high-precision rotational position alignment. However, if the wafer expands or contracts at least partially, a high alignment precision over the entire surface of the wafer cannot be obtained.
Meanwhile, in the TTL system, an alignment mark on a wafer can be detected for each small area which is exposed by projection through a projection lens. Therefore, a good alignment precision can be obtained over the entire surface of the wafer. However, with this system, alignment speed is slow and a rotational alignment error is hard to eliminate.